432 CHEMICAL ENGINEERING
able when necessary. The space at the top of the pipe around the couple may be plugged with asbestos or waste and covered with pitch to keep water away from the insulation. The scale of the indicator is set to read the mean temperature of the bottom of the tube. It is convenient to have an extra pair of compensating leads or an extra thermocouple with its junction at the bottom of the pipe to measure this temperature occasionally. Usually the compensating leads of a base metal couple are marked or are equipped with one way terminals, so that they are easily connected properly to the head of the couple. If reversed at the couple the leads will cause an error double the amount of the compensation. When compensating leads of a base metal couple are properly connected to the couple no deflection of the indicator is obtained by heating the head of the couple.
The high cost of platinum prevents the use of compensating leads of the same metal in the case of a rare-metal couple but inexpensive lead wires of copper and an alloy of nickel-copper are now available for use with the platinum-platinum 90, rhodium 10 couple. These lead wires do not compensate individually but taken together they compensate to within 5°C. for a variation of 200°C. in the couple-lead wire junctions. Since the compensating lead wires for the rare metal couple do not compensate individually both terminals on the head of the couple should be always as nearly as possible at the same temperature. The copper wire of the compensating leads is connected to the platinum-rhodium wire of the couple and the copper-nickel wire is connected to the platinum wire of the couple, i.e., alloy wire to pure metal in each case. The cold junction is then located at the indicator end of the compensating leads. The temperature of this junction may be controlled if necessary by one of the methods described above. Copper wires are carried from this point to the indicator.
Many other methods are employed, either with or without compensating leads, for automatically correcting for the cold junction temperature. Leeds & Northrup use several different systems with their potentiometric indicators. Cold junction compensation can be accomplished over a limited range by use of a "shunted couple/' The Cleveland Instrument Co. uses a Whea'tsone bridge one arm of which contains a nickel coil located in the head of the couple. These methods are discussed by Foote, Harrison and Fairchild in a paper given at the Chicago Symposium of the American Institute of Mining Engineers, September, 1919, on pyrometry.1
Thermocouple Installations.—The installation of a large thermocouple equipment requires the services of competent electricians. Just as much attention, if not more, should be given to the wiring, switches, switchboards, etc. as is given in the case of ordinary power installations. Proper fixtures should be used to mount the couple in the furnace. Lead wires should have a weather proof covering and should be run in a metal conduit except for a short length of flexible cable at the ends of the conduit. The conduit should be grounded to prevent leakage from power installations or lighting circuits. All joints in the lead wires should be soldered and taped. When indicators or recorders of low resistance are employed it is of the greatest importance to have a well constructed electrical installation to insure a constant line resistance. Since instruments of low resistance are usually calibrated for a low line resistance of definite value the size of copper wire required for a long line may be as large as No. 12 or No. 10. Frequently switches rated at 100 amp. are required although the actual thermoelectric current is only a few milliamperes. If the indicator is of high resistance or operates upon the potentiometric or semi-potentiometric principle the factor
1 This has been issued as a separate volume by the Institute.